Scientists assemble the XENON1T dark matter detector in the Gran Sasso Underground Laboratory in Italy. UChicago physicist Luca Grandi and his research group played a key role in preparing and assembling the xenon detector. Credit: XENON1T Collaboration

There is five times more dark matter in the universe than "normal" matter—the atoms and molecules that make up the familiar world. Yet, it is still unknown what this dominant dark component actually is. On Nov. 11 an international collaboration of scientists inaugurated the new XENON1T instrument designed to search for dark matter with unprecedented sensitivity at the Gran Sasso Underground Laboratory in Italy.

Dark matter is one of the basic ingredients of the universe, and searches to detect it in laboratory-based experiments have been conducted for decades. However, until today dark matter has been observed only indirectly, via its gravitational interactions that govern the dynamics of the cosmos at all length-scales. It is expected that dark matter is made of a new, stable elementary particle that has escaped detection so far.

"We expect that several tens of thousands of dark matter particles per second are passing through the area of a thumbnail," said Luca Grandi, a UChicago assistant professor in physics and a member of the Kavli Institute for Cosmological Physics. "The fact that we did not detect them yet tells us that their probability to interact with the atoms of our detector is very small, and that we need more sensitive instruments to find the rare signature of this particle."

Grandi is a member of the XENON Collaboration, which consists of 21 research groups from the United States, Germany, Italy, Switzerland, Portugal, France, the Netherlands, Israel, Sweden and the United Arab Emirates. The collaboration's inauguration event took place Nov. 11 at the Laboratori Nazionali del Gran Sasso, one of the largest underground laboratories in the world.

"We need to put our experiment deep underground, using about 1,400 meters of solid rock to shield it from cosmic rays," said Grandi, who participated in the inauguration along with guests from funding agencies as well as journalists and colleagues. About 80 visitors joined the ceremony at the laboratory's experimental site, which measures 110 meters long, 15 meters wide and 15 meters high.

There, the new instrument is installed inside a 10-meter-diameter water shield to protect it from radioactive background radiation that originates from the environment. During introductory presentations, Elena Aprile, Columbia University professor and founder of the XENON project, illustrated the evolution of the program. It began with a 3 kilogram detector 15 years ago. The present-day instrument has a total mass of 3,500 kilograms.

Fighting against radioactivity

XENON1T employs the ultra-pure noble gas xenon as dark matter detection material, cooled down to –95 degrees Celsius to make it liquid.

"In order to see the rare interactions of a dark matter particle in your detector, you need to build an instrument with a large mass and an extremely low radioactive background," said Grandi.

"Otherwise you will have no chance to find the right events within the background signals."

For this reason, the XENON scientists have carefully selected all materials used in the construction of the detector, ensuring that their intrinsic contamination with radioactive isotopes meet the low-background experiment's requirement.

"One has to realize that objects without any radioactivity do not exist," Grandi explained. "Minute traces of impurities are present in everything, from simple things like metal slabs to the walls of the laboratory to the human body. We are trying to reduce and control these radioactive contaminants as much as possible."

The XENON scientists measure tiny flashes of light and charge to reconstruct the position of the particle interaction within their detector, as well as the deposited energy and whether it might be induced by a dark matter particle or not. The light is observed by 248 sensitive photosensors, capable of detecting even single photons. A vacuum-insulated double-wall cryostat, resembling a gigantic version of a thermos flask, contains the cryogenic xenon and the dark matter detector.

The xenon gas is cooled and purified from impurities in the three-story XENON building, an installation with a transparent glass facade next to the water shield, which allows visitors to view the scientists inside. A gigantic stainless-steel sphere equipped with pipes and valves is installed on the ground floor.

"It can accommodate 7.6 tons of xenon in liquid and gaseous form," said Aprile. "This is more than two times the capacity we need for XENON1T, as we want to be prepared to swiftly increase the sensitivity of the experiment with a larger mass detector in the near future."

Aiming for a dark matter detection

Once fully operational, XENON1T will be the most sensitive dark matter experiment in the world. Grandi's group has been deeply involved in the preparation and assembly of the xenon Time Projection Chamber, the core of the detector. His group is also in charge for the development of the U.S. computing center for XENON1T data analysis via the UChicago Research Computing Center, directed by Birali Runesha, in close cooperation with Robert Gardner and his team at the Computation Institute.

In addition to Columbia's Aprile, leading the other six U.S. institutions are Ethan Brown, Rensselaer Polytechnic Institute; Petr Chaguine, Rice University; Rafael Lang, Purdue University; Kaixuan Ni, University of California, San Diego; and Hanguo Wang, University of California, Los Angeles.

XEON1T's first results are expected in early 2016. The collaboration expects the instrument to achieve most of its objectives within two years of data collection. The researchers then will move their project into a new phase.

"Of course we want to detect the dark matter particle," Grandi said, "but even if we have only found some hints after two years, we are in an excellent position to move on as we are already now preparing the next step of the project, which will be the far more sensitive XENONnT."

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28 comments

Good luck finding something that has never been observed, and exists only in people's imaginations today. I am not saying that dark matter absolutely does not exist, but it seems we are spending an enormous amount of effort on something that may well yield nothing in return. When are we going to stop this madness?

Good luck finding something that has never been observed, and exists only in people's imaginations today. I am not saying that dark matter absolutely does not exist, but it seems we are spending an enormous amount of effort on something that may well yield nothing in return. When are we going to stop this madness?

When it is understood that the universe doesn't structure itself via gravity.

"When it is understood that the universe doesn't structure itself via gravity."

The universe like every other machine can not structure itself in any way. The order (information) in the system comes only by intelligent intervention. Like every other machine is needed of certain minimal number of parts and systems to be possibly to function.

The universe is programmed by the Creator. The 3D structure of the vacuum which filled the space can be reprogrammed locally or globally in any moment by the will of the Creator . It control the behavior of elementary particles and their interactions with each other and electromagnetic waves.

I wonder why nobody seems to have considered a rather obvious source for the phenomenon called DM?It seems that every photon of light has a component of mass or mass-equivalent (if it is affected by mass in, say, gravitic lensing).Yet we can only detect photons when they are coming directly at us (or at our measuring instrument).Only a tiny fraction of the photons emitted by the Sun come our way, so there are zillions of photons whizzing out into space we do not detect. There are zillions upon zillions of photons whizzing past Earth at all times that we do not detect.Our universe is full of photons whizzing about in all directions and we can only detect those radiations that come our way! Is this the elusive DM?

Instead of looking for dark matter one should examine anti matter, if no antimatter is found within the universe, and we are created from an opposite material, possibly dark matter is this source.

Antimatter wave functions, as far as I know, are equivalent to matter wave-functions. For example, an anti-electron is identical to an electron, except its charge is positive. An anti-proton is identical to a proton, except its charge is negative. Therefore, we would expect the anti-hydrogen atom to also have the same orbitals and energy levels. Hence, antimatter would have the same spectral properties as its matter counterpart and we could expect to detect it as we would expect to detect ordinary matter.

We would also likely detect significant amounts of radiation (I believe gamma rays) coming from regions of antimatter as it came into contact with random particles of matter.

Instead of looking for dark matter one should examine anti matter, if no antimatter is found within the universe,

Lots of positrons. According to Feynman positrons are electrons moving backward in time. If so, antimatter created at the Big Bang will be hard to find since there was no time before the big bang. Anyway, antimatter created at the Big Bang would have no elapsed time in this universe. So it's not very likely we will find any of this antimatter.

Lots of positrons. According to Feynman positrons are electrons moving backward in time.

Positrons are not actually electrons moving backward in time (as far as we know). The time symmetry of the equations of electromagnetism means that positrons can be modeled as electrons. It's a mathematical convenience. Positrons and electrons are still, however, subject to the 2nd law of thermodynamics, which means that both move through time in the usual direction.

If so, antimatter created at the Big Bang will be hard to find since there was no time before the big bang.

Even if it were true that animatter created at the BB moved backwards in time, you would still be wrong. Feynman and Wheeler modeled positrons as electrons moving back through time FROM THE END OF TIME. Hence, positrons created during the BB would really just have been time-reversed electrons destroyed during the BB. We would still detect them.But that's all moot, as anti-matter is not DM.

Yep. There is no such thing as DM particles, only entropic gravity due to naturally occurring variations in spacetime curvature.

Well, that's an interesting hypothesis (which, I must add, has not been demonstrated in the least bit), but how does that differ from a Modified Gravity theory? We know DM reliably clumps around baryonic matter, so what you are effectively arguing is that the EFEs are only approximations, as, now the Ricci curvature tensor would no longer completely depend on the stress-energy tensor. That is modified gravity.

Also, I am not sure what you mean when you say "entropic." How would the gravity be entropic? Are you saying it arises from thermodynamic principles? This seems to conflict with the idea that it is due to naturally occuring variations in space-time.

Not provable? Can you give an example for the order emerged due to random events

"who is making the snowflakes?"

The physical conditions on Earth created by the Creator. It is interesting to know that human made snowflakes have no this unique and beautiful shape like natural snowflakes. The Creator asked Job several thousand years ago "do you entered the treasury of snow?", when people have no idea of their beautiful crystalline structure.

Here's one credited to my thermodynamics professor:Imagine you live in California, and that you like to eat M&Ms. So, you have a flat bottomed bowl filled with M&Ms in a single layer (because you've eaten most of them). Now, a small earthquake happens. After the earthquake whatever random configuration the M&Ms were in before, they will have assembled themselves into a quasi-periodic hexagonal lattice.

I find that creationists have this cartoon understanding of entropy, probably because most only learn about entropy in gases.(tbc)

(continued)Because ideal gases are so well behaved, their equations of entropy are particular well behaved. I suspect this is why introductory thermodynamics courses use them to introduce the concepts of entropy to students. Unfortunately, this also creates the impression that entropy is synonymous with an increase in disorder.

However, this is only true because the particles that make up ideal gases do not interact, except for elastic collisions.

Now, entropy, has a few equivalent definitions, but the one I prefer is the negative log of the probability of its macrostate. A macrostate is basically just a property of the entire system. Temperature is a familiar example of macrostates, but crystalline structure is also a macrostate.A microstate is a particular configuration of particles that yields that macrostate. Each macrostate is caused by many microstates,(to be continued)

(continued)The shape of the sand in the sand box is also macrostate, but if you zoom in, there are many particle configurations that yield that shape. These configurations are the microstates. In the case of sand, the sand castle has a much lower entropy than if the sand were just to fill up the entire box.

However, again, that's because sand particles only weakly interact.

Now, consider another system: a closed suit-case full of clothes. If you've ever packed a suitcase, you've seen that it is easier to pack if you fold the clothes first, rather than haphazardly throw them in there randomly. Now, if you consider all configurations of, say, 20 articles of clothes in a suit case, you will find that microstates with the clothes folded neatly are more common than those with the clothes haphazardly arranged. Hence, states with higher entropy are LIKELIER to be ordered.

This is how higher states of entropy can be MORE ORDERED than lower states.

@viko_mxActually, let me make something clear. It isn't just interaction between constituent particles that tend to make ordered states more likely than disordered ones. It's actually the CONSTRAINTS on a system.

For example, gravity constrains ordered arrangements on particles on earth. If you consider a volume without gravity, weakly interacting particles will be fairly randomly and uniformly distributed throughout the volume. Gravity often forces the density of particles to increase in the direction of the field.

And if the constituent parts interact, and can combine to form larger interacting parts, then you likely will get a feedback loop. That's because the original constraint will place the particles into a slightly ordered configuration, which will randomly create larger constituent parts, which will then introduce more constraints that will organize those larger constituent parts into even larger constituent parts with further constraints, and so on...

Now, if you consider all configurations of, say, 20 articles of clothes in a suit case, you will find that microstates with the clothes folded neatly are more common than those with the clothes haphazardly arranged. Hence, states with higher entropy are LIKELIER to be ordered.

This is how higher states of entropy can be MORE ORDERED than lower states.

Now, if you consider all configurations of, say, 20 articles of clothes in a suit case, you will find that microstates with the clothes folded neatly are more common than those with the clothes haphazardly arranged. Hence, states with higher entropy are LIKELIER to be ordered.

This is how higher states of entropy can be MORE ORDERED than lower states.

What does folding clothes have to do with configurations?

The arrangement of your clothes in a suitcase represents a microstate. The macrostate would be the number of clothes that are neatly folded.

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